database-driven web sites, second edition1 chapter 3 introduction to client-side scripts

Database-Driven Web Sites, Second Edition

1

Chapter 3

INTRODUCTION TO CLIENT-SIDE SCRIPTS


2

Objectives

In this chapter, you will:

• Learn how to reference objects in HTML documents using the HTML Document Object Model and dot syntax

• Learn how to create and debug client-side scripts that use JavaScript methods, event handlers, and custom functions

• Create and manipulate JavaScript variables

• Create and use JavaScript built-in objects


3

Objectives

In this chapter, you will:

• Learn how to use JavaScript global functions to perform data type conversions

• Become familiar with JavaScript decision control and looping structures

• Understand the differences between JavaScript and Java


4

Referencing HTML Document Objects

• To enhance Web pages, JavaScript program commands must be able to reference objects on a Web page

• JavaScript commands reference Web page objects using the HTML document object model (DOM)


5

Object-Oriented Concepts

• Object: abstract representation of something in the real world, with specific properties and actions

• Object class: defines the properties and actions of similar objects

• Class instance: object that belongs to a class• Event: an action that occurs within an object as a

result of a user action• Event handler: block of program commands that

executes when an event occurs


6

The HTML Document Object Model

• The HTML document object model (DOM):– Hierarchical naming system

– Enables a developer to reference and access HTML objects and their properties, methods, and events within JavaScript commands

• Current Netscape and Internet Explorer browsers support the basic DOM, first introduced in Netscape Navigator 2


7


• In the DOM currently used:– Window: top-level object class; represents a

browser window

– Child object classes within a window: history, document, and location

– A document object contains all of the elements, or child objects, on a Web page

– Primary child objects: link, form, and anchor


8



9

Referencing HTML Objects Using Dot Syntax

• Dot syntax: references an object in an HTML document based on its hierarchical location among the DOM HTML objects

• This hierarchy is called the object path


10

Dot Syntax Using Object Names

• An HTML link, form, or anchor object can be referenced using its object name in dot syntax as follows: window.document.object_name

• To reference a child element within a document form, a dot is placed after the form’s object_name and then the name of the form element is specified

• Once you specify the object path, you can then reference an object’s properties and call its methods


11

Dot Syntax Using Object IDs

• Object ID attribute: – alternate way to reference HTML objects in dot

syntax

– uniquely identifies an element within an HTML document

– can be used instead of the name attribute value when specifying the path to an object

• Any HTML tag can have an ID attribute value


12

Using the Visual Studio .NET IDE to Create a Client-Side Script

• IntelliSense lists can be used to provide choices in JavaScript commands

• The IntelliSense information lists available child objects, methods, properties, and events that can be used to complete HTML, dot syntax, and program statements

• Items within the IntelliSense lists have visual icons to specify the item type


13

Adding Script Tags to an HTML Document

• Client-side script can be created by enclosing JavaScript commands within the script tag

• JavaScript commands are usually enclosed in HTML comment tags


14


• JavaScript commands:– Are case-sensitive– Can span multiple lines in a text editor and HTML

file– End of each command is designated with a

semicolon (;)– Comment statements can be included

• The line signaling the end of the script must be prefaced by typing the JavaScript comment indicator (/) followed by the closing HTML comment tag (->)


15


• Script tags:– Can be placed almost anywhere in an HTML

document

– Should not be placed within document title tags or within a style tag because the script interpreter does not look for script tags in these locations

• Avoid nesting scripts within additional elements• A document can contain multiple sets of script tags,

however, you should enclose all script commands within a single script tag


16

JavaScript Methods

• An object has associated methods that:– Perform specific actions on the object

– Use the object in a way that affects the document or script

• Syntax to call a method:

document.obj_name .method_name(para1,para2,…) • If the method has no associated parameters, use

empty parentheses after the method name


17

JavaScript Methods

• The alert method opens a message box that displays a short message

• The text in an alert can reference and display properties of HTML form elements


18

JavaScript Methods

• Document methods create dynamic Web pages using client-side scripts

• Examples:– document.clear method clears the contents of the

current document

– document.write method adds new HTML tags and elements dynamically


19

JavaScript Functions

• Function: self-contained group of program commands that programmers call within a larger program

• Global functions: built-in functions that can be called from any JavaScript command

• Custom functions: programmers create custom functions to perform program-specific tasks


20

JavaScript Functions

• All function code should be placed in the heading section of the HTML document

• The commands that call the functions are placed where they need to be executed in the document body

• The command that calls a function may pass one or more parameters to the function

• Function commands may perform an action or return a data value to the calling command


21

Creating a Custom Function

• The first line of a function contains the function declaration, which defines the function name and specifies the parameters that the function receives from the calling program or command


22

Creating a Custom Function

• Function declaration: – Begins with the reserved word function

– Then the name of the function and an optional parameter list is specified

• The function name must begin with a letter, and can contain numbers, letters, and underscores (_)

• Function names cannot contain any other special characters, such as hyphens (-) or pound signs (#)

• Letters within function names are case-sensitive


23

Calling a Function

• A JavaScript function can be called from directly within a JavaScript command by specifying:– Name of the function

– List of parameter values that are to be passed to the function

– Syntax:

function_name (param1_value, param2_value, …)


24

Event Handlers

• HTML objects have events that occur as a result of user actions

• Event handlers:– Contain program commands that execute when an

event occurs

– Syntax

<element attributes

event_handler_name ="JavaScript_command ">


25

Event Handlers

• It is not a good practice to place JavaScript tags and commands at the end of the body section of an HTML document

• To execute a script when a browser first loads, an onload event handler associated with the HTML document is created, and this event handler calls a function or executes a command


26

Displaying Script Error Information In Internet Explorer

• When an error occurs in a client-side script, Internet Explorer displays a default error notification message

• For debugging client-side scripts, script developers usually configure Internet Explorer to display a Script Debugger Error dialog box, providing information about script errors


27

Configuring Internet Explorer to Display or Suppress Default Script Error Notification

Messages


28

Configuring Internet Explorer to Display or Suppress Default Script Error Notification

Messages

• Defaults on Advanced properties page:– Check the Disable script debugging check box

– Clear Display a notification about every script error

• Browser displays the error notification message and allows the user to retrieve additional information


29

Configuring Your Workstation to Display the Script Debugger Error

Message Box

• If the Display a notification about every script error check box is checked, Internet Explorer displays the Script Debugger message box only if Script Debugger is installed on the workstation


30

Using Variables in JavaScript Commands

• Programs use variables:– to store numbers, text, dates, and other types of

data that the browser can display and that client-side script commands can manipulate

• Variables have a name and data type that specifies the kind of data that the variable stores

• Data types enable:– Program error checking

– Optimization of memory


31

Using Variables in JavaScript Commands

• Strongly typed language: – requires programmer to declare variables and their

data types before they are used in a program command

• JavaScript is loosely typed: programmer does not need to specify the data type when the variable is declared


32

Declaring JavaScript Variables and Assigning Values to Variables

• Variable declaration syntax: var variable_name ;

• Variable names must begin with a letter, and can contain numbers, letters, and underscores (_)

• Letters within variable names are case-sensitive• To assign a value to a variable

variable_name = value;

• Can declare and initialize a variablevar variable_name = initial_value ;


33

Using JavaScript Operators to Manipulate Variables

• Operators perform arithmetic and string operations on literal and variable values

• Concatenation operator (+): joins two separate string elements into a single string element

• Display string values on multiple lines: use “\n” in a string to break its display into separate lines


34

Using JavaScript Operators to Manipulate Variables

• Assignment operators: allow programmers to perform operations and assignments in a single command

• Plus sign:– Is overloaded, which means it can be used for two

different operations

– Can be used for numeric addition and string concatenation


35

Specifying the Order of Operations

• Operations are evaluated in a specific order• The interpreter evaluates operations in

parentheses or square brackets first, and then evaluates additional operations in the order listed

• The interpreter evaluates assignment operations last


36

Specifying the Order of Operations

• Operations at the same level, such as addition and subtraction, are evaluated from left to right

• Concatenation operations are evaluated from left to right, with operations in parentheses evaluated first


37

Using JavaScript Built-In Object Classes

• To perform similar operations in JavaScript, built-in object classes are used

• To use a built-in object, create an instance and assign a value to the new object’s value property

• The object’s methods can then be used to perform tasks on the associated value

• Syntax to create a new object:

var variable_name = new object_type ();


38


• String Objects– Create a new String object named currentItem and

assign “3-Season Tents” to its value property:

var currentItem = new String();

currentItem.value = "3-Season Tents";

• Math Objects– The Math object class expands the usefulness of

the JavaScript arithmetic operators

– Object instances of the Math class do not need to be created


39


• Date Objects– Date objects format and manipulate date and time

values and retrieve the date and time on the workstation

– Date value is divided into individual year, month, day, current hour, minute, and second components

• Number Objects

– Number objects format numeric values


40

Using Global Functions to Perform Explicit Data Type Conversions

• By default, all data values that users enter into Web page forms are text strings

• To convert text strings to numbers, perform an explicit data type conversion

• To perform a conversion, write a program command to convert a value from one data type to another

• JavaScript provides global functions to perform explicit data type conversions


41

Converting Strings to Numbers

• parseInt() global function: – Converts a string representation of a number into a

number representation– Removes any decimal or fractional parts

• parseFloat() global function:– Converts a string representation of a number into a

number representation– Retains the decimal or fractional parts

• The general syntax for these functions is:number_variable = parseInt ("string_number ");number_variable = parseFloat ("string_number ");


42

Converting Numbers to Strings

• The easiest way to convert a date or number variable to a string data type is to concatenate the date or number variable to an empty string literal

• An empty string literal:– String value that does not contain any characters

– Consists of two double quotation marks, with no characters inserted in between: “”


43

Decision Control Structures

• Decision control structures: execute alternate statements based on true/false conditions

• “if” control structure tests whether a condition is true or false– If the condition is true, the interpreter executes a

set of program statements

– If the condition is false, the interpreter skips the program statements


44


• if/else control structure– Tests a condition

– Executes one set of statements if the condition is true, and an alternate set if the condition is false

• if/else if control structure allows the program to test for many unrelated conditions, and execute specific program statements for each true condition


45


• switch control structure: – Program can test multiple conditions that compare

the same variable value

– Executes faster than the equivalent if/else if structure

– Requires fewer program lines

• However, it can only be used when the condition evaluates whether an expression is equal to another expression


46

Using the AND and OR Logical Operators in Control Structure

Conditions

• AND operator (&&): overall condition is true if both conditions are true

• OR operator (||): overall condition is true if either condition is true


47

Creating Repetition (Looping) Structures

• Loop: – A repetition structure that processes multiple

values the same way– Repeatedly executes a series of program

statements and periodically evaluates an exit condition

• Pretest loop: evaluates the exit condition before any program commands execute

• Posttest loop: one or more program commands execute before the loop evaluates the exit condition the first time


48

Creating Repetition (Looping) Structures

• while loop: pretest loop • do while loop: posttest loop• for loop: counting loop

– Programmers declare and control a counter variable from within the loop structure


49

Contrasting JavaScript and Java

• Even though both JavaScript and Java use a C-style syntax for common programming tasks, their underlying structures and purposes are very different

• Java is a full-featured object-oriented programming language

• JavaScript is more limited and runs within HTML documents


50

Summary

• Programmers use client-side scripts for tasks such as validating user inputs entered on HTML forms, opening new browser windows, and creating cookies

• The HTML document object model (DOM) is a hierarchical naming system that enables scripts to reference browser objects

• DOM objects are accessed and manipulated using dot syntax containing either object name or id attribute values


51

Summary

• Events: actions that take place in a document as a result of a user action

• Functions: self-contained groups of program commands that are called within a script

• User-defined functions perform specific tasks • JavaScript is a loosely typed language• Decision control structures are created using if,

if/else, if/else if, and switch statements • Loops include while, do while, and for loops

database-driven web sites, second edition1 chapter 3 introduction to client-side scripts

Documents

locationa document object

object names

object pathplanning

anchor object

forms object

object ids object id

html link

document form